feat: add runner health admission checks (#1090)

Opt-in local task-admission checks under a `health_check` config section (disabled by default):

- pause new task fetching when free disk space on the workspace volume is below the configured minimum
- optional executable health-check script — a non-zero exit, timeout, or start failure marks the runner unavailable
- checks run only while the runner is idle; the last result is reused while a job is active, and polling resumes automatically on recovery
- `/readyz` reports task-admission readiness (reusing the poll loop's last check); `/healthz` stays a process-liveness endpoint

---------

Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1090
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>

internal/pkg/config/config_test.go

@@ -48,6 +48,41 @@ func TestLoadDefault_DefaultsWorkdirCleanupAge(t *testing.T) {
 	assert.Equal(t, 10*time.Minute, cfg.Runner.IdleCleanupInterval)
 }
 
+func TestLoadDefault_HealthChecksAreOptIn(t *testing.T) {
+	cfg, err := LoadDefault("")
+	require.NoError(t, err)
+	assert.False(t, cfg.HealthCheck.Enabled)
+	assert.Equal(t, int64(1024), cfg.HealthCheck.MinFreeDiskSpaceMB)
+	assert.Empty(t, cfg.HealthCheck.Script)
+	assert.Equal(t, 30*time.Second, cfg.HealthCheck.Interval)
+	assert.Equal(t, 10*time.Second, cfg.HealthCheck.Timeout)
+	assert.False(t, cfg.Metrics.Enabled)
+}
+
+func TestLoadDefault_DiskAndReadinessSettings(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "config.yaml")
+	require.NoError(t, os.WriteFile(path, []byte(`
+health_check:
+  enabled: true
+  min_free_disk_space_mb: 4096
+  script: /usr/local/bin/runner-health
+  interval: 15s
+  timeout: 3s
+metrics:
+  readiness_grace: 45s
+`), 0o600))
+
+	cfg, err := LoadDefault(path)
+	require.NoError(t, err)
+	assert.True(t, cfg.HealthCheck.Enabled)
+	assert.Equal(t, int64(4096), cfg.HealthCheck.MinFreeDiskSpaceMB)
+	assert.Equal(t, "/usr/local/bin/runner-health", cfg.HealthCheck.Script)
+	assert.Equal(t, 15*time.Second, cfg.HealthCheck.Interval)
+	assert.Equal(t, 3*time.Second, cfg.HealthCheck.Timeout)
+	assert.Equal(t, 45*time.Second, cfg.Metrics.ReadinessGrace)
+}
+
 func TestLoadDefault_UsesConfiguredWorkdirCleanupAge(t *testing.T) {
 	dir := t.TempDir()
 	path := filepath.Join(dir, "config.yaml")